The
To: header lists one or more of the recipients of
the mail message. Other headers, such as Cc:, also
list recipients.
If the header of a mail message lacks recipient information
(To:, Cc:, and
Bcc: header lines), pre-V8.7
sendmail added an
Apparently-To: header line and put the
recipient's address from the envelope into the field
of that header. Beginning with V8.7, the way a message with no
recipients is handled is determined with the
NoRecipientAction option (NoRecipientAction).
